It is highly desirable for tires to exhibit good traction characteristics on both dry and wet surfaces. However, it has traditionally been very difficult to improve the traction characteristics of a tire without compromising its rolling resistance and tread wear. Low rolling resistance is important because good fuel economy is virtually always an important consideration. With increasing fuel prices, tires that provide even lower rolling resistance will be demanded by consumers. Good tread wear is also an important consideration because it is generally an important factor that determines the life of the tire.
The traction, tread wear, and rolling resistance of a tire is dependent to a large extent on the dynamic viscoelastic properties of the elastomers utilized in making the tire tread. In order to reduce the rolling resistance of a tire, rubbers having a high rebound have traditionally been utilized in making the tire's tread. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in tire treads. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads.
For example, various mixtures of styrene-butadiene rubber (SBR) and polybutadiene rubber are commonly used in tread rubber for automobile tire treads. SBR is included in tire tread formulations primarily to improve the traction characteristics of the tire without greatly compromising tread-wear or rolling resistance.
Styrene-butadiene rubber synthesized by anionically-initiated polymerization (s-SBR) has emerged as a preferred type of SBR for tire treads, but there are many ways of preparing s-SBR. To that end, the physical properties of SBR, e.g., microstructure and molecular weight can be varied greatly depending on the process variables used during synthesis thereof. These process variables are numerous with generally unpredictable interplay there between when they are altered or manipulated. Such process variables include polymerization temperature, length of reaction time, number of reactors, types and amounts of reagents, and the amounts of monomer used, for example. As such, the resulting s-SBR is not at all likely to be universally satisfactory for every purpose and need encountered by tire treads for use in tires.
Accordingly, what is needed is a solution-polymerized heterogeneous rubbery polymer, e.g. a styrene-butadiene rubber, and a process for making same, wherein the resulting rubbery polymer is useful in tire tread formulations, which has a balanced combination of properties, particularly including desirable traction characteristics, wear resistance, and low rolling resistance.